The ranges of colors we perceive are the consequence of a mixture of light of different wavelengths. Certain wavelengths of blue, green, and red light, referred to as the "primary" colors, will produce a wider range of colors when combined with each other in varying intensities than any other three-color combination. Addition of two primary colors will produce the "secondary" colors cyan (green and blue), magenta (red and blue), and yellow (red and green). Mixture in the proper proportions of all three primary colors, or of a secondary color with its complementary primary color, gives white light.
The primary colors of light should be carefully distinguished from the primary colors of coloring agents or pigments. A primary pigment color absorbs and therefore substracts a primary color of light, and reflects or transmits the other two, since we see opaque objects by the light they reflect and transparent objects by the light they transmit. The primary pigment colors are the secondary light colors, and vice versa. Mixing the three pigment primaries in the correct combination will result in black, the total absorption of light.
A black-and-white image generally contains light and dark portions ranging from pure white to pure black, with intermediate intensity levels consisting of various shades of gray. The shades of gray or gray "levels" which make up such an image are processed by the human eye in a performance that can be characterized as relatively poor in comparison to the proficiency of the human eye in dealing with color. At a given point in an image the average observer can only differentiate about one to two dozen gray levels, whereas the same observer can discern thousands of variations in color. This far superior ability of the eye to discriminate subtle details of color images as opposed to black-and-white images has led to the development of image processing techniques that provide a black-and-white image with pseudocolors.
In pseudocolor techniques the processing starts with a monochrome image. Each small portion of the image, known as a "pixel," receives a color according to some scheme. Pseudocolor encoding assigns a color to each pixel based on some varying property of the image, and in so-called pseudocolor density encoding the property is the brightness of the image at the location of the pixel.
The use of pseudocolored images has found wide application, largely in the form of computer processed images. Pseudocoloring methods have typically been utilized in mapping analysis by NASA of the earth and other planets by earth satellites and space probes, in military intelligence studies of satellite reconnaissance pictures, in earth resources surveys, in medical diagnosis by internal bodily imaging methods, and in industrial inspection applications such as for defective parts in automated production.
There is a relatively small number of pseudocolorization methods which do not depend on computer processing. In one method, a halftone-encoded image transparency is illuminated by two superimposed laser beams of different wavelengths; this method is described in the article by H. K. Liu and J. W. Goodman entitled "A new coherent optical pseudocolor encoder," in the journal Nouvelles Revues Optique, volume 7, page 285, published in 1976. In another method, a halftone-encoded image transparency is illuminated by a collimated white-light beam, as described in the article by A. Tai et al. entitled "White light pseudocolor density encoder," in the journal Optics Letters, volume 3, page 190, published in 1978. By selective spatial filtering of the different color components contained in the various diffraction orders in the Fourier plane, a pseudocolor density encoded image can be obtained in the output plane.
Pseudocolor encoding can also be achieved with grating encodings, as described in the article by T. H. Chao et al. entitled "White light pseudocolor density encoding through contrast reversal," in the journal Optics Letters, volume 5, page 230, published in 1980; in the article by J. A. Mendez and J. Bescos entitled "Gray level pseudocoloring with three primary colors by a diffraction grating modulation method," in the Journal of Optics, volume 14, page 69, published in 1983; and in the article by F. T. S. Yu et al. entitled "White light pseudocolor encoding with three primary colors," in the Journal of Optics, volume 15, page 55, published in 1984. In these methods, a positive (original) image, a negative image, and a product of the two are sequentially contact printed through a grating onto black-and-white photographic film. The orientation of the grating varies with each exposure. For retrieval of the pseudocolor image, the encoded image transparency is illuminated with a white light plane wave. After color spatial filtering at the Fourier plane, each of three encoded image components passes through a primary color primary filter.
Although a broad band of pseudocolor can be obtained satisfactorily in the output plane by these encoding techniques, the fact that they are not real-time techniques has severely limited their applications. A real-time technique is one that can be carried out in so-called real time, without any delay required for processing. Photographic or computer-based methods necessarily involve some interval of delay to produce the pseudocolored image, and for certain applications any delay is unacceptable.
Several real-time density-encoding pseudocoloring methods have been reported. One technique utilizes a contrast-reversal spatial filter to obtain a negative image. This negative image is carried by a laser beam of one wavelength and superimposed with a positive image of a different wavelength. The mixture of these two images generates a color-coded image in real time. The technique is described in the article by J. Santamaria et al. entitled "White light pseudocolor density encoding through contrast reversal," in the journal Optics Letters, volume 10, page 151, published in 1979.
Pseudocolor encoding can also be achieved by a method based on the scattering properties of photographic film, as described in the article by J. A. Mendez and M. Nieto-Vesperinas entitled "Light scattering by film grain noise: application to gray level optical pseudocoloring," in Applied Optics, volume 22, page 2068, published in 1983. A positive, a negative, and a bidirectionally modulated image can be obtained simultaneously by illuminating an original image transparency with oblique transmission and diffuse reflection. Coding each of the three images with a primary color and superposing them produces a pseudocolor image.
Pseudocoloring has also been obtained directly by using the birefringent effect of a liquid crystal light valve spatial light modulator (LCLV SLM). Since the LCLV is biased at a specific frequency and voltage and is read out with a white light beam, the output image is automatically color coded. A similar effect has been reported with an LCTV SLM in the article by F. T. S. Yu et al. entitled "Real-time pseudocolor encoding using a low-cost liquid crystal television," in the journal Optical Laser Technology, volume 19, page 45, published in 1987.